This invention relates in general to the field of rework processes for surface mount technology, and in particular a method for reapplying a solder mask material for areas of solder mask that are removed during the component rework process such as those for a Ball Grid Array (BGA).
Printed Circuit Boards (PCB""s) are manufactured using various additive and subtractive process. The circuitry is generated by starting with a core panel of copper laminated to at least one side of a core material. Generally, the copper is laminated to both sides of the core material. The copper is generally chemically etched using photodeveloping processes and various chemicals to create the circuitry. Several cores are then laminated to create a multilayer PCB. The several cores are drilled in predetermined locations and later plated to create electro-mechanical connections between the circuits of the various layers, commonly referred to as vias. Solder resist or mask is applied to each outer layer of the laminated PCB, over the bare copper.
Solder mask is required to control the flow of solder and contain the solder to the desired solder pads, respective to each component. This is particularly critical for Ball Grid Array (BGA) devices, whereby the solder pads are attached to a via. The solder is preapplied to the BGA in predefined volumes. During reflow, the solder liquifies and flows around the receiving pads on the PCB. Should the solder mask be removed, when attaching a replacement component, the solder can flow from the receiving pad down the via, resulting in a defect. Defects can include open circuits, mis-registered components, shorts, and reliability risks.
Conventional methods for reworking surface mount components, require heat to remove the component previously reflowed to a Printed Circuit Board (PCB). The heating process degrades the adhesion between the solder mask and the bare copper circuitry. Further, any residual solder needs to be removed from the receiving pads. This is generally completed by placing solder wick (pre-fluxed, braided copper strands woven into a ribbon) against the pads and heating the wick using a soldering iron. The solder wick is known to abrade and remove the solder mask from the PCB. Solder may also flow under the resist and cause the resist to lift from the copper.
The solder mask process is generally applied to the PCB during the PCB fabrication process. The process utilized phototooling and photodeveloping systems. These systems require expensive equipment and a flat surface. These two factors make replacement of the solder mask on a populated assembly at an assembly shop impractical.
It is known to use micro stencils, such as U.S. Pat. No. 5,107,759, Omori, et al. dated Apr. 28, 1992 to apply solder to solder pads for replacement of solder during the rework process.
It is known to use look up/look down prism systems during the alignment process of various solder printing and component placement processes. One known example is taught by Freeman, U.S. Pat. No. 4,924,304, issued May 8, 1990.
Thus, what is necessary is a low cost and efficient method and apparatus for replacing solder mask around pads on a component or on a substrate, where the solder mask has been removed during a rework process.
One aspect of the present invention is to provide a low cost tool for replacing solder mask or a similar type material onto exposed metallization of a component or PCB.
A second aspect of the present invention is the ability to apply a pattern of solder mask to a desired area, leaving the desired solder pads exposed.
A third aspect of the present invention is a solder mask applicator to apply the pattern of solder mask to the desired area.
A fourth aspect of the present invention is the use of a foil to define the desired pattern.
A fifth aspect of the present invention is the use of a rubber stamp to define the desired pattern.
A sixth aspect of the present invention is the inclusion of a mechanism to aid in registration of the pattern on the solder mask applicator and the receiving site.
A seventh aspect of the present invention is the use of a prism, look up/look down system to align the pattern of the solder mask applicator and the receiving site.
An eighth aspect of the present invention is the use of a predetermined depth, solder mask reservoir to apply the solder mask to the solder mask applicator.
A ninth aspect of the present invention is the incorporation of the present invention into a component rework apparatus, preferably one which comprises a means to provide alignment, a means to control relational proximity between the selective solder mask applicator and the plurality of receiving pads, and a controllable means to apply heat as required.
A tenth aspect of the present invention is the inclusion of a means to at least partially automate the processes described herein, such as including a means to apply a predetermined set of instructions to an automated means.